Josephson tunnel junctions and other types of superconducting tunnel junctions are becoming popular for various applications, such as memory cells and logic circuitry. Various materials have been investigated for junction superconductors and junction insulators. Niobium is commonly used for junction superconductors.
Niobium (Nb) is a non-critical material (the Earth's crust contains 20 ppm Nb in pyrochlore and other ores) with excellent mechanical stability. It also causes fewer diffusion problems and has a high transition temperature in comparison, for example, to lead-lead junctions. However, niobium junctions often suffer from poor quality of the insulator and oxidation of niobium used for superconductors. Specifically, forming niobium superconductors often cause degradation of insulators. The insulators are often made from oxides, and oxygen from these insulators can go into the niobium superconductors, thereby causing their oxidation. This often leads to increased single particle currents or excess currents in the voltage range below the gap voltage. As a result, the junction has a large spread in Josephson current density.